1. Field of the Invention
The present invention relates to a control system for controlling the engagement of a clutch and the speed ratio of a continuously variable transmission, and more particularly to a control system for electrically controlling the engagement of a clutch and the speed ratio of a continuously variable transmission.
2. Description of the Prior Art
Continuously variable transmissions such as belt-type continuously variable transmissions have been proposed in the past, and have already found use in automobiles or the like. The belt-type continuously variable transmissions typically comprise a metallic V-belt trained around drive and driven movable pulleys whose width is variable. The belt-type continuously variable transmissions are often required to have a neutral state, which is achieved by a clutch that is disposed in the power transmission path from an input shaft to an output shaft for selectively transmitting engine power from the input shaft to the output shaft.
The speed ratio of the belt-type continuously variable transmission is controlled when the widths of the drive and driven pulleys vary. The clutch is controlled to select the neutral state and also to start moving the automobile. To control the widths of the drive and driven pulleys, the belt-type continuously variable transmission has a hydraulic cylinder for imparting a lateral pressure to the movable pulley cone of each of the pulleys. The lateral pressure applied to the movable pulley member by the hydraulic cylinder is controlled to vary the effective pulley diameters for thereby controlling the speed ratio. The control of the lateral pressure applied to the pulleys is also effective to control the tension of the V-belt. One known control system for controlling a belt-type continuously variable transmission is disclosed in Japanese Laid-Open Patent Publication No. 61-206862, for example.
In the case where a belt-type continuously variable transmission of the above structure is incorporated in an automobile, the lateral pressure applied to the pulleys and the engagement of the clutch are controlled depending on the throttle opening and rotational speed of the engine of the automobile which actuates the belt-type continuously variable transmission. It is possible to generate hydraulic pressures for controlling the pulleys and the clutch using an intake vacuum of the engine and a governor valve, and to employ the generated hydraulic pressures to control the pulleys and the clutch. However, use of such hydraulic pressures makes the control system complex due to hydraulic valves that are needed, and fails to perform sophisticated control operation.
Frequently, the above controlling hydraulic pressures are generated based on detected electric signals representing the throttle opening and rotational speed of the engine. Japanese Laid-Open Patent Publication No. 63-215437, for example, discloses a control system for controlling a belt-type continuously variable transmission with controlling hydraulic pressures generated on the basis of electric signals.
The controlling hydraulic pressures are generated using solenoid-operated valves and an electrically operated actuator based on the electric signals. In the event of an electric failure that cuts off the supply of electric energy to the solenoid-operated valves and the electrically operated actuator, the solenoid-operated valves and the electrically operated actuators fail to operate, and the belt-type continuously variable transmission cannot be controlled.